Metal-oxide nanoparticles offer large potential for electronic applications. Materials include e.g. titanium-oxides (BaTiO3, SrTiO3, TiO2, PbZrTiO3, etc.). A key characteristic property of these materials is their high relative permittivity ∈r (for example, ∈rTiO2˜80, ∈rSrTiO3˜300, ∈rBaTiO3˜1000).
Of particular interest are also the ferroelectric properties for some of these materials: for example, the spontaneous polarization and piezoelectricity (e.g. BaTiO3) [L. Huang et al., Barium titanate nanocrystals and nanocrystal thin films: Synthesis, ferroelectricity, and dielectric properties, J. Appl. Phys. 100, 034316 (2006); S. Ray et al., Direct observation of Ferroelectricity in Quasi-Zero-Dimensional Barium Titanate Nanoparticles, small 2, 1427 (2006)], and the tunable dielectric permittivity (e.g. SrTiO3, BaxSr1-xTiO3).
Another characteristic property of metal-oxides is their high material stability (e.g. high melting temperatures, hardness, stability against ambient air, etc.).
However, there exist several problems in realization of practical devices based on metal-oxide nanoparticles. As an example, let us consider the realization of a parallel plate capacitor where the middle insulating layer (between the well-conducting electrodes typically of metal) is intended to be made of the metal-oxide nanoparticles. The metal-oxide nanoparticles, deposited on the first electrode, tend to form a porous structure, which is prone to formation of electrical shorts when the second electrode is deposited. Furthermore, the mechanical stability of the metal-oxide nanoparticle layer is typically not good; in particular, the interparticle adhesion and the adhesion to the electrodes are typically inadequate.
To avoid the above problems, nanoparticles can be embedded in an insulating matrix (filler). The matrix material provides the insulation property (i.e. prevents electrical shorts) and the adhesive properties. However, due to application of the matrix material, a series capacitance is created between the particle and the well-conducting electrodes. This series capacitance is highly undesirable as it e.g. screens the embedded nanoparticle from the electric field applied between the electrodes. As one of the consequences, the effective capacitance of the structure can be drastically reduced. The problem is well illustrated e.g. in the case of a ferroelectric memory cell. The relative permittivity of the insulating “filler” material is typically substantially smaller than for the high-epsilon ferroelectric nanoparticle, thus causing a substantial potential drop already at small thickness. Depolarizing field is induced which limits, for example, the polarization stability (decreased memory retention time). From another viewpoint, the insulating property (required for preventing shorts between the electrodes) hinders the charge flow over the series capacitance, which can cause substantial delay in transferring the applied voltage to actually occur across the nanoparticle. A further complication arises for memory readout: the state of the ferroelectric memory cell is conventionally read by applying an electric pulse to the memory capacitor (opposite pulse polarity causes polarization reversal and related charge pulse is detected; parallel polarity does not cause the effect). The high impedance and the potentially low conductance of the series capacitance can induce drawbacks for an effective pulse readout.
The present invention is also related to other types of non-volatile memory. In particular, the phase change materials, also known as chalcogenide, or ovonic materials, have emerged as a potential candidate for high-density storage. The chalcogenide material is typically electrically heated (above 600° C.) to induce the phase transformation. Based on the electrical exposure (i.e. the intensity of the heating pulse), the material phase can be alternated between crystalline and amorphous. The two phases typically exhibit at least an order of magnitude difference in their electrical conductivity. The stored bit is thus coded in the electrical conductivity of the memory cell, and written electrically.
A central problem in the chalcogenide memories is to obtain sufficiently large current density j (A/m2) to induce the phase transformation In practice, this is typically translated into obtaining a sufficiently small electrode area. The minimum linewidths using printing fabrication methods (e.g. gravure or inkjet printing) are typically of the order of tens of micrometers (orders-of-magnitude larger than the state-of-the-art IC-fabrication process linewidths). Thus, reaching the required current densities becomes impractical e.g. in the standard sandwich (parallel-plate) memory cell configuration.
WO 2007/030483 discloses a method of using nanotube elements in the form of non-woven nanotube fabric as heating elements in memories.
US 2007/0045604 discloses a chalcogenide-based programmable conductor memory device. A conductive nanoparticle (e.g. tungsten, titanium nitride, platinum, palladium, ruthenium) is provided on the surface of an electrode for achieving a high electrical field strength in the vicinity of the nanoparticle for achieving a narrow current channel in a memory material. One of the drawbacks in this design is that the particle has to be in good electrical contact with the electrode, which makes the fabrication of the device inconvenient. Thus, there is a need for novel types of structures and methods providing for relaxed manufacturing tolerances, in particular with respect to the positioning accuracy of the electrodes and the nanoparticles.